The present invention relates generally to ground proximity warning systems and methods for use in aircraft and marine applications and, more particularly, to ground proximity warning systems, methods and computer program products for controllably altering the base width of an alert envelope to account for uncertainties associated with the position of the aircraft or ship.
An important advancement in aircraft flight safety has been the development of ground proximity warning systems. These warning systems analyze the flight parameters of the aircraft and the terrain surrounding the aircraft. Based on this analysis, these warning systems provide alerts to the flight crew concerning possible inadvertent collisions of the aircraft with surrounding terrain or other obstacles.
Although these warning systems are quite useful in providing the flight crew with information concerning potential problems with the navigation of the aircraft, the usefulness of these systems must be balanced against problems associated with the generation of nuisance alarms. Specifically, although it is advantageous to provide the flight crew with as much information as possible concerning terrain and obstacles in the flight path of the aircraft, this information should be delivered to the flight crew in a timely manner, such that the flight crew will pay close attention to the information. If alarms are provided too far in advance to the flight crew concerning terrain that is still far away from the present position of the aircraft, the flight crew may become desensitized to the alarms and may potentially ignore alarms from the ground proximity warning system altogether. Further, the generation of alarms concerning terrain that is still far away from the present position of the aircraft may also add stress and confusion on the flight crew and may overshadow other more critical alarms in the cockpit.
For this reason, at least one ground proximity warning system defines an alert envelope and, more particularly, both a caution envelope and a warning envelope. The imaginary alert envelope moves with the aircraft and is constructed to extend generally forwardly of the aircraft and to define a region in which alerts will be generated if terrain or other obstacles enter by penetrating the alert envelope. In this regard, U.S. Pat. No. 5,839,080 to Hans R. Muller, et al. and assigned to AlliedSignal, Inc. describes an advantageous ground proximity warning system that generates an alert envelope. The contents of U.S. Pat. No. 5,839,080 are hereby incorporated by reference in their entirety.
As described by U.S. Pat. No. 5,839,080 and depicted in FIGS. 1 and 2, an alert envelope is defined by a number of parameters, including a look ahead distance (LAD), a base width (DOFF) and a terrain floor (H). In general terms, the look ahead distance defines the distance in advance of the aircraft that the alert envelope extends. Similarly, the terrain floor typically defines a vertical distance below the aircraft which is utilized during the construction of the floor of the alert envelope. Further, the base width is the lateral width of the alert envelope at a location proximate the aircraft. In one example, the base width of an alert envelope extends laterally 0.125 nautical miles to each side of the aircraft for a total base width of 0.25 nautical miles with the aircraft centered relative to the base width.
While the base width may be a constant value, at least one ground proximity warning system reduces the base width as the aircraft nears an intended runway on which the aircraft will land. By way of example, this ground proximity warning system reduces the base width in a linear manner from a maximum of 0.25 nautical miles to a minimum of 0.04 nautical miles as the aircraft goes from 4 nautical miles from the intended runway to 2 nautical miles from the intended runway. By reducing the base width of the alert envelope, the ground proximity warning system reduces the overall size of the alert envelope and therefore requires the terrain or other obstacles to be closer to the aircraft, at least in a lateral direction, prior to generating an alert. Since an aircraft that is approaching a runway for a landing may be positioned more closely to terrain or other obstacles as the aircraft follows the desired glideslope than while enroute, the reduction in the base width of the alert envelope reduces the generation of disconcerting nuisance alerts during the landing phase, while still providing alerts for terrain or other obstacles that are actually in the flight path of the aircraft.
As shown in FIG. 2, an alert envelope is therefore at least partially defined by a center tine 10 and a pair of outer tines 12. During relatively level flight in which the absolute value of the roll angle of the aircraft is a relatively small value, such as less than 5xc2x0, the center tine points along the ground track of the aircraft. In instances in which the aircraft has a larger roll angle, such as greater than 5xc2x0, the center tine is angularly displaced from the ground track by an angle approximating the roll angle to. thereby incorporate a lead angle during turning of the aircraft. In either situation, the outer tines generally diverge from the center tine by a predetermined angle. Thus, the alert envelope is somewhat broader for locations in advance of the aircraft than at the current position of the aircraft. As shown in FIG. 2, the spacing between the outer tines at a location proximate the current position of the aircraft is defined by the base width, such as 0.25 nautical miles in one example, and the outer tines diverge from the center tine at a predetermined angle, such as 3xc2x0, in advance of the aircraft.
As described by U.S. Pat. No. 5,839,080, the ground proximity warning system can construct a pair of alert envelopes, namely, a caution envelope and a warning envelope. While each envelope has a similar shape as described above and depicted in FIGS. 1 and 2, the caution envelope typically extends further ahead of the aircraft than the warning envelope and is therefore larger than the warning envelope. Accordingly, the ground proximity warning system will generate cautionary alerts in instances in which the upcoming terrain or other obstacles penetrate the caution envelope, but not the warning envelope. Once the upcoming terrain or other obstacles penetrate the warning envelope, however, the ground proximity warning system will generate a more severe warning alert. As such, a pilot can discern the severity of the alert and the rapidity with which evasive maneuvers must be taken in order to avoid the upcoming terrain or other obstacles based upon the type of alert that is provided, i.e., a cautionary alert or a more severe warning alert.
As will be apparent from FIG. 2 and from the foregoing description of the alert envelope generated by a ground proximity warning system, the alert envelope is at least partially dependent upon the current location of the aircraft. In this regard, the base width of the alert envelope is centered laterally relative to the current position of the aircraft and the base width, in turn, defines the origin of the outer tines of the alert envelope. Unfortunately, the current position of the aircraft cannot be determined with absolute certainty. Instead, the current position of the aircraft is always subject to at least some uncertainty.
The uncertainty associated with the determination of the current position of the aircraft depends upon a number of factors, including the type of navigation system utilized by the aircraft and, in some instances, the phase of the flight, i.e., final approach, terminal area, or enroute. For example, a global positioning satellite (GPS) system can determine the current position of the aircraft with very little uncertainty, irrespective of the phase of the flight. For example, one GPS system can determine the current position of the aircraft to within an uncertainty of 0.054 nautical miles irrespective of the phase of the flight.
Unfortunately, other navigation equipment cannot determine the current position of the aircraft as exactly. For example, a flight management system is another common type of navigation equipment that is capable of receiving input from a variety of other navigational instruments, such as inertial navigation instruments, radio navigational instruments including very high frequency omnidirectional radio range (VOR) systems, and GPS systems, and that generates position information therefrom. Depending upon the navigational instrument that provides input to the flight management system, the current position provided by the flight management system may have a rather substantial uncertainty with the uncertainty increasing as the aircraft is further from the intended runway, that is, the uncertainty for an aircraft enroute exceeds the uncertainty for an aircraft in the terminal area which, in turn, exceeds the uncertainty for an aircraft in final approach. In this regard, a flight management system that relies upon a VOR system for position information generally has the largest uncertainty with respect to the determination of the current position of the aircraft. For example, the AC 20-130 airworthiness approval of multi-sensor navigation systems for use in U.S. national airspace system (NAS) and Alaska defines 95% probability numbers for the uncertainty of a flight management system that relies upon a VOR system to be 2.8 nautical miles enroute, 1.7 nautical miles in the terminal area and either 0.3 or 0.5 nautical miles during final approach. With respect to the two uncertainty values that are provided for the final approach phase, the larger uncertainty is associated with a flight management system that only receives position information from a VOR system, while the smaller uncertainty value is associated with a flight management system that receives position information from a VOR system that works in conjunction with distance measuring equipment.
In instances in which the uncertainties associated with the determination of the current position of the aircraft are relatively large, the aircraft may not be centered laterally relative to the base width of the alert envelope, but may actually be positioned at a location offset from the center tine 10 of the alert envelope. In these instances, it is possible that the alert provided by the ground proximity warning system may not provide the pilot of the aircraft with as much reaction time as desired to respond to the alert and take appropriate evasive action, especially if the aircraft is actually positioned nearer to the terrain or obstacle that penetrated the alert envelope then the current position of the aircraft would indicate. Accordingly, while ground proximity warning systems have proven to be a great advance in aircraft flight safety, it would be desirable to further refine the alert envelopes generated by ground proximity warning systems to accommodate uncertainties associated with the position of the aircraft.
The ground proximity warning system, method and computer program product of the present invention controllably alter the base width of the alert envelope in order to accommodate uncertainties associated with the current position of the aircraft. In one embodiment, the ground proximity warning system, method and computer program product controllably alter the base width of the alert envelope depending upon an error value, i.e., an uncertainty, associated with the current position of the aircraft. Since the error value associated with the current position of the aircraft is largely dependent upon the type of navigation equipment onboard the aircraft, the ground proximity warning system, method and computer program product of another embodiment controllably alter the base width of the alert envelope depending upon the type of navigation equipment that provides the current position of the aircraft. As such, the ground proximity warning system, method and computer program product define a more refined alert envelope that will provide a pilot with ample time to respond even if the current position of the aircraft is slightly errant.
According to the present invention, an alert envelope is generated, typically by a processor and/or computer readable program code means executed by the processor. The alert envelope defines an alert region extending at least forward of an aircraft. The alert envelope is based at least partially upon a base width proximate the current position of the aircraft and extending laterally relative to the aircraft. The alert envelope is also at least partially based upon a look-ahead distance that extends forward of the aircraft.
The ground proximity warning system, method and computer program product of one embodiment alters the base width of the alert envelope depending upon an error value associated with the current position of the aircraft. For example, the base width of the alert envelope is made smaller as the error value associated with the current position of the aircraft decreases. Conversely, the base width of the alert envelope is made larger as the error value associated with the current position of the aircraft increases. The ground proximity warning system, method and computer program product of this embodiment is also responsive to a terrain database having a plurality of different resolutions. For example, the terrain database typically has an increased resolution near airports and highly populated regions and a lower resolution for those regions remote from an airport or highly populated regions. As such, the ground proximity warning system, method and computer program product can also alter the base width of the alert envelope based upon the resolution of the terrain database at the current position of the aircraft. In one advantageous embodiment, for example, the base width of the alert envelope is altered to be at least as large as the difference between the error value associated with the current position of the aircraft and 50% of the resolution of the terrain database at the current position of the aircraft.
Typically, the error value associated with the current position of the aircraft is dependent upon the type of navigation equipment that provides the current position of the aircraft and the distance of the aircraft to the intended runway. As such, the ground proximity warning system, method and computer program product of one embodiment of the present invention alters the base width of the alert envelope depending upon the type of navigation equipment that provides the current position of the aircraft. In this regard, the base width of the alert envelope is typically larger if the current position of the aircraft is provided by a flight management system than if the current position of the aircraft is provided by a global positioning system. Due to the relatively small uncertainty associated with the position determination of a global positioning system, the base width of the alert envelope can be limited to a predetermined maximum value in instances in which a global positioning system is providing the current position of the aircraft. Since the error or uncertainty associated with the position determination provided by navigation equipment such as a flight management system can be at least partially dependent upon the phase of flight, the ground proximity warning system, method and computer program product can also alter the base width of the alert envelope based upon the distance of the aircraft from the intended runway.
Therefore, the ground proximity warning system, method and computer program product of the present invention controllably alter the base width of the alert envelope in order to compensate for errors associated with the current position of the aircraft, such as the errors that are dependent upon the type of navigation equipment that provides the current position of the aircraft and/or the distance of the aircraft to the intended runway. In addition, the ground proximity warning system, method and computer program product of one embodiment of the present invention also alter the base width of the alert envelope based upon the resolution of the terrain database at the current position of the aircraft. Accordingly, the ground proximity warning system, method and computer program product ensure that alerts provided by the ground proximity warning system in response to terrain or other obstacles penetrating the alert envelope are provided far in advance of the terrain or other obstacles such that the pilot has a sufficient opportunity to assess the situation and take proper evasive action, even in instances in which the aircraft is actually displaced from the current position identified by the navigation equipment in a direction toward the upcoming terrain or other obstacles.